Capacitive proximity sensors have been developed to enable touch-less or non-contacting object detection. For example, the sensor may be configured as an elongate strip and affixed to a moveable panel or the frame of a door or window opening to detect the introduction of foreign objects in the door or window path. Sensors of this type are disclosed in the U.S. Pat. Nos. 6,700,393, 6,723,933, 6,750,624 and 6,777,958 to Haag et al., assigned to Delphi Technologies, Inc., and incorporated herein by reference.
In general, capacitive proximity sensors have two conductive plates and a detection circuit that detects changes in the capacitance between the plates since objects in proximity to the sensor change the intensity of fringing electric fields between the plates. For example, the aforementioned U.S. Pat. No. 6,777,958 discloses a detection circuit in which the sensor capacitance determines the frequency of an RF oscillator, and a detection circuit measures changes in that frequency by mixing the output of the oscillator with a fixed frequency signal and low-pass filtering the result. Regardless of how the change in capacitance is detected, it is important to account for the influence of factors such as sensor moisture, temperature and aging of the sensor materials. Also, it is important to be able to provide multiple sensor elements without incurring the expense of a separate detection circuit for each of the sensor elements. Accordingly, what is needed is an improved detection apparatus that is configured to operate with one or more capacitive proximity sensors and that is insensitive to sensor moisture and other environmental and aging factors that influence the capacitance of the sensor.